Large screen displays have recently become of interest for the purpose of showing movies at home, presentations, and the like. Therefore, many types of projection apparatus using light valves have been proposed; among these apparatus, a liquid crystal projection apparatus has recently been commercialized which displays images on a large screen by enlarging and projecting images displayed in a miniaturized liquid crystal display panel by a projection lens or the like.
Liquid crystal display panels electrically convert the optical conditions of the panels to display images; there are different types of operation principles for the liquid crystal display panels. For example, a twisted nematic (TN) liquid crystal display panel used for presently commercialized liquid crystal projection apparatus utilizes the rotatory polarization of liquid crystals which is caused by the application of an electric field to the panel. However, in order to modulate light, the TN liquid crystal display panel requires polarizers for the incident surface as well as the output surface (surface of outgoing radiation) of the panel, so that the panel has poor efficiency in utilizing light.
On the contrary, liquid crystal display panels which form optical images in response to a change in light scattering conditions in the panels do not require a polarizer. Phase change (PC) liquid crystal display panels, dynamic scattering mode (DSM) liquid crystal display panels, polymer dispersion liquid crystal display panels, and the like are examples of these liquid crystal display panels. Among these panels, polymer dispersion liquid crystal display panels of the type disclosed in U.S. Pat. No. 4,435,047 have been enthusiastically researched in expectation of an improvement in brightness. Polymer dispersion liquid crystal display panels have advantages in not requiring a polarizer and an orientation treatment.
In liquid crystal display panels using polarizers, the light absorbed at the polarizers is mostly converted into heat. Therefore, it is difficult for these liquid crystal display panels to display images with intense brightness by increasing the power of the light source. In addition, since the polarizers and the liquid crystal display panels are heated to a high temperature by absorbed light, radiated heat and the like, the properties of the polarizers and panels are degraded after a short period.
Coating of the alignment layer and a rubbing treatment are required for manufacturing the TN liquid crystal display panel. The rubbing treatment not only adds to manufacturing time and labor but can tear the thin-film transistors (TFT) on the panel by static electricity, which results in lowering yield and increasing the manufacturing cost of the panel. Moreover, as the number of picture elements of a liquid crystal display panel used for a liquid crystal projection type television increases to more than 300,000, the size of each picture element becomes minute. When the number of picture elements in the TN liquid crystal display panel increases, more signaling wires and TFTs having uneven surfaces are provided to the picture elements. As a result, the surface of the TN liquid crystal display panel becomes uneven, and it is therefore difficult to carry out the rubbing treatment on the TN liquid crystal display panel.
Considering the dispersing conditions of liquid crystals in polymer, there are roughly two types of polymer dispersion liquid crystals. Drops of liquid crystals dispersed irregularly in polymer is one of these types, called polymer dispersion liquid crystals (PDLC). Another type of polymer dispersion liquid crystals is called polymer network liquid crystal (PNLC): a network of polymer is spread out in a liquid crystal layer as if liquid crystals are regularly absorbed in a sponge. Liquid crystal display panels including these types of polymer dispersion liquid crystals display images by controlling the scattering and transmitting conditions of light in the panels.
Thermoplastic resins or thermosetting resins may be used as polymers to disperse PDLC as long as they are transparent. Among these resins, moreover, it is most preferable to use a resin of an ultra-violet curing type as the polymer, since a conventional method of manufacturing a TN mode liquid crystal display panel can be directly applied. The conventional method comprises the steps of setting two substrates formed with electrodes by a spacer to maintain the gap between them and to have the electrodes face each other, fixing the substrates with epoxy resin (sealing material) to form a vacant cell, and injecting liquid crystals dispersed in a resin (liquid crystal solution) into the vacant cell.
When this conventional method is applied to manufacture a polymer dispersion liquid crystal display panel, an ultra-violet curing type acrylic resin is particularly useful as the resin in which to disperse liquid crystals. For example, when the resin is blended with liquid crystals to prepare a liquid crystal solution, the solution has enough fluidity to be injected at room temperature. After irradiating the injected liquid crystal solution with light, the resin is cured, phase-separating the liquid crystals. As a result, a polymer dispersion liquid crystal layer is formed.
The operation of dispersing liquid crystals in a polymer is explained by referring to FIGS. 20 (a) and 20 (b). In these figures, 201 is an array substrate; 202 is a picture element electrode; 203 is a counter electrode; 204 is a drop of liquid crystals; 205 indicates a polymer; and 206 is a counter electrode. A TFT is connected to picture element electrode 202 (not shown in FIGS. 20 (a) and 20 (b)). The drops of liquid crystals 204 are oriented irregularly as shown in FIG. 20 (a) when the TFT is off and voltage is not applied to picture element electrode 202. As a result, the refractive index of the drop of liquid crystals 204 becomes different from the refractive index of polymer 205 so that irradiated light is scattered. FIG. 20 (b), on the other hand, shows a polymer dispersion liquid crystal panel when a TFT is on and voltage is applied to picture element electrode 202. As shown in FIG. 20 (b), the liquid crystals are oriented in one direction. If the refractive index of the liquid crystals after the orientation is the same as the refractive index of the polymer, irradiated light transmits through array substrate 201 without being scattered. When the liquid crystals are in a drop condition as PDLC, the diameter of a drop is called an average particle diameter. The average diameter of a PNLC, on the other hand, is called an average pore.
FIG. 21 shows an example of a conventional liquid crystal projection apparatus using a polymer dispersion liquid crystal display panel. Light emerging from a lamp 211 is focused by a concave reflector 212, and enters a liquid crystal display panel 213. If the light is not modulated in liquid crystal display panel 213, all of the light then enters a projection lens 214. Liquid crystal display panel 213 is a polymer dispersion liquid crystal display panel, in which a liquid crystal layer 218 is sandwiched between glass substrates 216 and 217. On the inner surfaces of glass substrates 216 and 217, picture element electrodes in a matrix condition are applied.
The scattering conditions of light vary in response to image signals applied to the liquid crystal display panel, so that optical images are formed in liquid crystal display panel 213 due to a change in the light scattering conditions. When voltage is applied to a picture element, light emerging from the light source transmits through the liquid crystal display panel and is projected by way of projection lens 214 onto screen 219. Thus, a bright optical image is displayed on screen 219. Light from the light source is scattered by the liquid crystal display panel if voltage is not applied to the picture element. The scattered light neither enters projection lens 214 nor reaches screen 219, thus displaying a dark optical image on screen 219. Optical images formed by liquid crystal display panel 213 in response to changes in light scattering conditions are enlarged and projected by way of the projection lens to screen 219. The collection angle of light going out from liquid crystal display panel 213 and entering the projection lens is determined by the pupil diameter of projection lens 214.
The quantity of light within a fixed solid angle of light outgoing from a liquid crystal display panel is influenced by the light scattering conditions of the liquid crystal display panel. For example, as the light scattering increases, the quantity of light within a fixed solid angle decreases. A conventional liquid crystal projection apparatus having an aperture inside the projection lens can provide a bright projection image on a screen; and its composition is simple. However, this conventional liquid crystal projection apparatus has a poor contrast ratio between the brightness of a white display and the brightness of a black display. As one solution to this problem, the collection angle of the projection lens is minimized when the apparatus shows a black display on a screen with a liquid crystal display panel in a light scattering state. As a result, the quantity of scattered light transmitting through the projection lens and reaching a screen decreases, and the brightness of the black display declines. As a result of this decrease in the brightness of the black display, the contrast raio increases. As another solution, light may be scattered more intensely by the liquid crystal display panel to decrease the quantity of light within a solid angle of light going out from the panel as well as the quantity of scattered light transmitting through the projection lens, thus reducing the brightness of the black display on the screen and improving the contrast ratio.
When light is completely scattered by a liquid crystal display panel, the contrast ratio (CR) is calculated from the formula, 1/sin.sup.2 .sigma., wherein .sigma. is the half of a collection angle (half angle) of the projection lens. (See Dewey.; Proc. of SID.; p. 138; 1977.) The most preferable contrast ratio of a direct vision type apparatus having an aperture in the projection lens is more than 30:1, while the most preferable contrast ratio of a projection type apparatus having an aperture in the projection lens is more than 100:1.
It is necessary to intensify the light scattering conditions of a liquid crystal display panel in an apparatus having an aperture in the projection lens so that the contrast ratio is increased. When a polymer dispersion liquid crystal display panel is used as a light valve, it is required that the panel functions with a low driving voltage and has high light scattering properties. By increasing the thickness of the polymer dispersion liquid crystal layer in the panel, the light scattering properties will improve. However, the driving voltage is also required to be high, so that driving the TFT of the polymer dispersion liquid crystal panel becomes difficult.
Especially when the apparatus is a projection type apparatus, the F number of a projection optical system adjusted to a light source including a metal halide lamp and a concave reflector is from 4 to 5. Thus, even if a polymer dispersion liquid crystal display panel in a complete light scattering state is used for the projection type apparatus, its contrast ratio is only on the order of 90:1.